1. Field of the Invention
This invention relates to a superconductive energy reversible storage circuit for alternating current power systems.
2. Description of the Prior Art
It is desirable that a nuclear power plant or large-scale steam-power station be controlled to run at a steady output. On the other hand, in general, demand for power markedly decreases at night. In order to avoid such unbalance between supply and demand of power, there has been a requirement for developing power storage equipment. Under the circumstance, one such equipment drawing attention is a superconductive energy storage system with a superconductive magnet.
There has already been known a superconductive energy storage system, which was disclosed in U.S. Pat. No. 4,122,512, Peterson et al. FIG. 7 illustrates the circuitry of this system. In FIG. 7 reference numeral 1 designates an alternating current input terminal connected to an AC power supply line; 2 a three-phase AC/DC reversible converter circuit consisting of thyristors; 3 a superconductive coil short-circuit switch; 4 a superconductive energy storage coil; 5 a refrigeration system for the superconductive energy storage coil 4; 21 a commutation reactance of the three-phase AC/DC conversion circuit; 22 a thyristor bridge circuit of the three-phase AC/DC reversible converter circuit 2; and 23 a phase control circuit for controlling energy flow through three-phase AC/DC reversible converter circuit 2.
The mode of operation will be described below:
Referring to FIG. 7, a reversible converter acting as three-phase AC/DC reversible converter circuit 2 is connected at the AD input terminal 1 to an energy source, and the phase control circuit 23 controls the flow of energy between a three-phase AC supply line and the superconductive energy storage coil 4 by adjusting the phase difference between AC supply voltage and the circuit current.
The circuit current delivers energy from a power supply line to the superconductive energy storage coil 4 when it has a delayed phase compared with the power supply voltage, and releases energy in the opposite direction when its phase is adjusted to be advanced.
When set to zero power-factor control, energy remains stored by the superconductive energy storage coil 4. Besides it is possible to separate coil current through the superconductive energy storage coil from the AC supply by closing of the superconductive coil short-circuit switch 3 which results in bypassing the thyristor bridge circuit 22.
Also reference numeral 5 indicates a refrigeration system for refrigerating the superconductive energy storage coil 4. The commutation reactance 21 is applied in the case where the thyristor bridge circuit 22 is a line commutated converter.
Because of the construction of the superconductive energy storage circuit so that current through the energy storage coil flows through the three-phase AC/DC reversible converter circuit and then the three-phase AC supply line, the current ratings of the AC supply line equipment and the thyristor converter had to be designed to be a maximum value of coil current. Owing to this, the current capacity of the AC-side parts of the superconductive coil energy storage circuit did not correspond to a service power established on the basis of the current rating of the coil, inevitably reflecting in large-scale loss. This is a problem encountered in the prior art.